1. Field of the Invention
The present invention relates to an electrostatic powder coating method for a road wheel, in particular, to a method for masking the inside of a bolt hole of a hub bolt.
2. Description of the Related Art
A road wheel used in an automobile is made from steel or aluminum and generally has applied thereto a surface coating for improving weather resistance and corrosion resistance. As a surface coating method, there has been employed an electrostatic powder coating method where a powder is applied on a substance to be coated by utilizing static electricity since such a method can provide a higher quality coating without the risk of polluting the environment compared with a conventional method of using an organic solvent.
As shown in FIGS. 7 and 8, a wheel for an automobile has a plurality of bolt holes 2 arranged on the same circumference around a center hole 1. As shown in FIG. 9, the wheel can be attached on an axle hub of an automobile (not illustrated) by inserting a hub bolt 3 protruding from the axle hub into a bolt hole 2 from the rear side of the wheel and screwing a hub nut 4 on the hub bolt 3 from the front side of the wheel. At this time, a tapered surface 5 formed inside the bolt hole 2 of the wheel contacts with a tapered surface 6 of the hub nut 4, thereby the axes of the bolt hole 2 and the hub bolt 3 can be aligned.
Therefore, for the accurate alignment of the bolt hole 2 and the hub bolt 3, masking is required during coating so as not to form a coating film 7 on the tapered surface 5 in the bolt hole 2.
A conventional coating method using such a masking technique is shown in FIGS. 10A to 10D. As shown in FIG. 10A, a wheel is placed on a supporting jig 8 and a disk-like mask 9 is inserted in a bolt hole 2 from the front side of the wheel so as to close the upper part of the tapered surface 5. In this state, as shown in FIG. 10B, a powder 11 is applied on the surface of the wheel with a coating machine 10. At this time, the powder 11 is accumulated also on the mask 9 as well. A baking treatment is then applied after eliminating the powder 11 accumulated above the tapered surface 5 by lifting away with the mask 9 as shown in FIG. 10C. The mask 9 taken out from the bolt hole 2 is air-blown by using an air nozzle 12, as shown in FIG. 10D, to remove the powder 11 for the next coating operation.
Another conventional coating method is shown in FIGS. 11A to 11C. As shown in FIG. 11A, a wheel is placed on a supporting jig 8. In this state, as shown in FIG. 11B, a powder 11 is applied on the surface of the wheel with a coating machine 10. Since the tapered surface 5 in the bolt hole 2 is not masked, the powder 11 is accumulated unnecessarily on the tapered surface 5. As shown in FIG. 11C, a suction nozzle 13 is then inserted into the bolt hole 2 from the front side of the wheel, that is, from above to eliminate the unnecessary powder 11 accumulated on the tapered surface 5 through the suction nozzle 13 by vacuuming. A baking treatment is thereafter applied.
According to these conventional methods, the coating can be conducted without forming a coating film on the tapered surface 5. However, the method shown in FIGS. 10A to 10D involves the risk of fall-off of the powder 11 from the mask 9 at the time of lifting the mask 9. The method shown in FIGS. 11A to 11C involves the risk of fall-off of the suctioned powder 11 from the tip of the suction nozzle 13 since the unnecessary powder 11 is removed by the suction nozzle 13 inserted into the bolt hole 2 from above the wheel. The fall-off of the powder 11 from the mask 9 or the suction nozzle 13 onto the tapered surface 5 results in an unnecessary coating film formation, or the fall-off onto another part of the wheel results in deterioration of the coating quality of the part. Further, since the powder 11 is applied or eliminated for the unnecessary portion in a state that the wheel is placed on the planar supporting jig 8 so that the opening at the lower part of the bolt hole 2 is closed by the supporting jig 8, the powder 11 remains in the lower part of the bolt hole 2 so that there is a risk of adhesion of the powder 11 onto the inner wall of the bolt hole 2.